That setup would effectively give you 2 sets of dampeners and I doubt thats what you want. Also, given the weight of the motor, and the length of that (effectively a torque arm) plate you are going to more than likely shear something. Either make an adapter that goes from those mounts (that are shot now, by the way) to the 4 bolt engine mount on the RB or make an adapter from the single bolt RB isolator to the crossmember.
I ended up ditching the crossmember all together and bolted my engine mounts to the frame rails.
-dan
edit: Perhaps I overlooked one thing, is the RB isolator single bolt on both sides? In that case you could bolt to the engine mount - but you are still applying a lot of twisting torque.